This anomaly consists of the hypertrophy of valves similar to membranes. Those valves are localized in the posterior urethra of male fetuses and determine different levels of urethral obstruction. The most frequent finding is the distended bladder with thick and trabeculated walls linked up with dilation of the prostatic urethra. All others urethral segments have normal caliber. There is a frequent association with vesicoureteral reflux. Dilated pyelocalyceal systems and tortuous ureters can be observed. The posterior urethral valves is one of the most frequent cause of fetal ascites. There is an association of 20 percent with specific chromosomal anomalies, especially trisomies 13, 18 and 21. In the most severe cases, pronounced oligohydramnios is detected, which leads to secondary pulmonary hypoplasia.